No its not. Video playback with an audio lag of several seconds? Not good.

You're confusing synchronization and latency. There's no particular reason you can't buffer up a few seconds of audio, yet make sure that audio is played exactly when the video calls for it.

For many uses you can (and should) buffer few secs of audio and video, but there are definitely cases where it's not possible. One case for example is realtime audio effects. You get audio stream from audiocard input, do processing and send stuff to audiocard output. You just can't have latency of more than few ms between input and output. So no buffering